Fix crash when pressing undo after assigning an IFC class to an object. Baby steps to solving the "Undo Sync".

src/blenderbim/blenderbim/bim/module/root/operator.py

@@ -76,6 +76,7 @@ class ReassignClass(bpy.types.Operator):
 class AssignClass(bpy.types.Operator):
     bl_idname = "bim.assign_class"
     bl_label = "Assign IFC Class"
+    bl_options = {'REGISTER', 'UNDO'}
     obj: bpy.props.StringProperty()
     ifc_class: bpy.props.StringProperty()
     predefined_type: bpy.props.StringProperty()

src/blenderbim/blenderbim/bim/module/sequence/prop.py

@@ -150,7 +150,7 @@ class BIMWorkScheduleProperties(PropertyGroup):
 
 
 class BIMTaskTreeProperties(PropertyGroup):
-    # This belongs by itself for performance reasons.
+    # This belongs by itself for performance reasons. https://developer.blender.org/T87737
     # In Blender if you add thousands of tasks it makes other property access in the same group really slow.
     tasks: CollectionProperty(name="Tasks", type=Task)